This invention relates to a method of injection molding contact lens molds for cast molding contact lenses having a rotationally asymmetric lens surface, and apparatus for carrying out the method.
One method in practice for making contact lenses is cast molding. Cast molding of contact lenses involves depositing a curable mixture of polymerizable monomers in a mold cavity formed by two mold sections, curing the monomer mixture, and disassembling the mold assembly and removing the lens. Other processing steps, for example, hydration in the case of hydrogel lenses, may also be employed. One mold section forms the anterior lens surface (anterior mold section), and the other mold section forms the posterior lens surface (posterior mold section). Prior to the cast molding of the contact lens, each of the mold sections is formed by injection molding a resin in the cavity of an injection molding apparatus. Mounted in the injection molding apparatus are tools for forming the optical surfaces on the mold sections. Whereas the mold sections are typically used only once for casting a lens, the injection molding tools are used to make hundreds of molds.
Several known cast molding methods have the potential to mold a finished contact lens, for example, U.S. Pat. No. 5,271,875 (Appleton et al.). Since these methods avoid time-consuming and labor-intensive operations such as lathing, the methods have been found to offer the potential to reduce production time and cost for the manufacture of spherical contact lenses.
However, various problems have been encountered in employing cast molding technology for manufacturing other types of contact lenses, especially contact lenses that have at least one rotationally asymmetric surface. As one example, toric contact lenses (i.e., contact lenses having a toric optical zone that are used to correct refractive abnormalities of the eye associated pith astigmatism) have at least one surface that is not rotationally symmetric. The problems encountered may be due to several factors. First, the toric optical zone is not spherical. Second, toric contact lenses include some type of ballast (such as prism ballast or slab-off zones) to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism; in order to provide such ballast, the edge thickness of the lens is not uniform about the entire circumference of the lens. As another example of such lenses, many mulitfocal designs are not rotationally symmetric.
Applicant found that, in forming contact lens molds for molding lenses having a rotationally asymmetric lens surface, problems were encountered in consistently obtaining contact lens molds having the same geometries. Such inconsistencies in the contact lens mold geometries translated to inconsistencies in cast molding contact lenses in the molds. The present invention solves this problem.